This invention relates to an apparatus and method for handling web in such a way as to minimize web transport inertia and consequent unwanted variations in web elongation.
The nature of the prior art over which the present invention represents an improvement is exemplified by U.S. Pat. No. 2,758,541, Tison.
In the Tison patent, a reciprocating motion is superimposed upon the otherwise continuous movement of the web from an unwind station to a rewind station (or other downstream operation). The result is to synchronize web movement with printing elements supported on base rollers which have a larger periphery than the minimum necessary to accommodate the circumferential extent of the printing and support elements. When the latter printing and support elements come around again to carry out their operation, the superimposed motion factor ensures that the web movement and position correspond to those of the printing and support elements.
To achieve this matching of speed and position, Tison provides a transport carriage which is directly driven intermittently parallel to the general direction of web movement, the latter""s continuous velocity being determined by the activity of draw cylinders. For example, immediately after the printing and support elements complete their contact with the web, the transport carriage slows down, then is driven through zero speed, then is retracted in the opposite direction in order to reset the position of the web vis a vis the printing and support elements on the next pass.
The type of mechanism just described is in general use. For example, it is utilized to do intermittent flat die cutting by moving the roller transport carriage in such a way as to stop the web for cutting.
Unfortunately, this design is severely limited in its registration accuracy, i.e., the accuracy that can be achieved in the stepping of the web in synchronism with rotation of the base cylinders, especially as running speeds and indexing rates are increased.
Because of the mechanical inertia of the carriage and the rollers it supports (linear inertia) and the rotational inertia of the rollers about which the web is entrained, registration accuracy tends to be poor, even at low indexing speeds (up to 30 or 40 indexes per minute). It is certainly not practical in the present day, since high accuracy is required from zero up to over 200 indexes per minute.
The fact that the carriage-mounted rollers must be accelerated and decelerated as the carriage moves through its cycle causes tension variations to be imparted to the web by virtue of the rollers"" inertial effects on the web.
Attention is further drawn to U.S. Pat. No. 4,009,814, issued Mar. 1, 1977 to Singh and U.S. Pat. No. 4,356,946, issued Nov. 2, 1982 to Gaskell. Both of these patents utilize a very large intermediate carriage which transports a plurality of toothed wheels and is rigged for translational movement toward and away from headers above and below it, upon which are mounted stationary toothed wheels. An endless chain or the like is strung between the various toothed wheels in a boustrophedonic configuration (festooning). The chain, carriage and wheels are all driven from outside (i.e., there is no individual drive to the wheels involved in the xe2x80x9cendlessxe2x80x9d configuration). As a result, the inertia that must be overcome from the outside is enormous. It consists of translational inertia in the carriage and the chain, translational inertial in the wheels, and rotational inertia in the wheels (i.e., resistance to a change in rotational speed).
In view of the shortcomings of the prior art, as set forth above, it is an object of one aspect of this invention to convert substantially continuous web motion, without objectionable tension fluctuations, into an intermittent motion, while ensuring that the web is in an accurate register position with respect to the elements (printing plates or die-cutting cylinders, for example) which are intended to carry out an operation on the web.
More particularly, this invention provides, for use in an apparatus for handling web, a cylinder module providing for the accumulation and de-accumulation of web at a controlled web strain and minimal inertia, the module comprising:
a frame,
a first main cylinder mounted on the frame for rotation about a first axis fixed with respect to the frame,
a first nip cylinder cooperating with the first main cylinder to form
a first nip to grip the web,
a first motor and controller adapted to drive the first main cylinder in a substantially continuous manner,
a second main cylinder mounted on the frame for rotation about a second axis fixed with respect to the frame, the second axis being substantially parallel with the first axis,
a second motor and controller adapted to drive the second main cylinder in a substantially non-uniform manner,
a third main cylinder mounted for constrained rotation about a third axis which is and remains parallel with the first and second axes while undergoing movement in a direction perpendicular to said first and second axes,
whereby the web can pass partially around the periphery of the first cylinder and through the first nip, thence to and partially around the periphery of the third cylinder, thence to and partially around the periphery of the second cylinder and through the second nip, the cylinders being disposed such that there is a straight web span from the third to the first cylinder which is substantially parallel to a straight web span from the third to the second cylinder, and
endless belt means associated with each end of the third cylinder for ensuring that the third cylinder, from moment to moment, automatically takes up an axis location and a rotational position which substantially control the web stress as it passes through the cylinders.
This invention further provides a method of handling web, utilizing a cylinder module providing for the accumulation and de-accumulation of web at controlled web strain and minimal inertia, the module comprising:
a frame,
a first main cylinder mounted on the frame for rotation about a first axis fixed with respect to the frame,
a first nip cylinder cooperating with the first main cylinder to form a first nip to grip the web,
a first motor and controller adapted to drive the first main cylinder in a substantially continuous manner,
a second main cylinder mounted on the frame for rotation about a second axis fixed with respect to the frame, the second axis being substantially parallel with the first axis,
a second nip cylinder cooperating with the second main cylinder to form a second nip to grip the web,
a second motor and controller adapted to drive the second main cylinder in a substantially intermittent manner,
a third main cylinder mounted for rotation about a third axis which is and remains parallel with the first and second axes while undergoing movement in a direction perpendicular to said first and second axes, said method comprising the steps:
passing the web partially around the periphery of the first cylinder and through the first nip, thence to and partially around the periphery of the third cylinder, thence to and partially around the periphery of the second cylinder and through the second nip, the cylinders being disposed such that the web span from the third to the first cylinder is substantially parallel to the web span from the third to the second cylinder,
utilizing endless belt means associated with each end of the third cylinder for ensuring that the third cylinder, from moment to moment, automatically takes up an axis location and a rotational position which substantially relieve the web of stress as it passes through the cylinders.
said endless belt means comprising:
a carriage at either end of the third cylinder to which said third cylinder is mounted for rotation, and for each carriage a guideway means ensuring that the carriage undergoes substantially rectilinear movement along a pathway which generally bisects the span between the first and second cylinders, each carriage supporting a first toothed wheel for rotation adjacent the third cylinder, the two first toothed wheels being locked together for simultaneous rotation,
the frame supporting, at opposed ends of the cylinders, two second toothed wheels for rotation on one side of the pathway and two third toothed wheels for rotation on the other side of the pathway, the two second toothed wheels being locked together for simultaneous rotation, and the two third toothed wheels also being locked together for simultaneous rotation,
the first, second and third cylinders each having, at either end, a coaxial toothed wheel fixed to the respective cylinder, and, at each end of the cylinders, an endless toothed belt;
the method further comprising: entraining said endless toothed belt around the toothed wheel of the first cylinder, thence along a first path to and around the toothed wheel of the third cylinder, thence along a second path to and around the toothed wheel of the second cylinder, the first and second paths being parallel, thence along a third path to and around said second toothed wheel, thence along a fourth path to and around said first toothed wheel, thence along a fifth path to and around the third toothed wheel, the fourth and fifth path being substantially parallel to each other and to the first and second paths, thence along a sixth path which returns it to said toothed wheel of said first cylinder.